In recent years, surface-treated steel sheets made by imparting rust resistance to base steel sheets, particularly galvanized steel sheets and galvannealed steel sheets, have been widely used in fields such as automobiles, home appliances, and building materials. In view of the improvement of automotive fuel efficiency and the improvement of automotive crash safety, there are increasing demands for lightweight high-strength automobile bodies made from automobile body materials having high strength and reduced thickness. Therefore, high-strength steel sheets are being increasingly used for automobiles.
In general, galvanized steel sheets are manufactured in such a manner that thin steel sheets manufactured by hot-rolling and cold-rolling slabs are used as base materials and base steel sheets are recrystallization-annealed and galvanized in an annealing furnace placed in a continuous galvanizing line (hereinafter referred to as CGL). Galvannealed steel sheets are manufactured in such a manner that alloying is performed after galvanizing.
Examples of the type of the annealing furnace in the CGL include a DFF (direct fired furnace) type, a NOF (non-oxidizing furnace) type, and an all-radiant tube type. In recent years, CGLs equipped with all-radiant tube-type furnaces have been increasingly constructed because the CGLs are capable of manufacturing high-quality plated steel sheets at low cost due to ease in operation and rarely occurring pick-up. Unlike DFFs (direct fired furnaces) and NOFs (non-oxidizing furnaces), the all-radiant tube-type furnaces have no oxidizing step just before annealing and therefore are disadvantageous in ensuring the platability of steel sheets containing oxidizable elements such as Si and Mn.
In a method for manufacturing a hot-dipped steel sheet made from a high-strength steel sheet containing large amounts of Si and Mn, PTLs 1 and 2 disclose a technique in which a surface layer of a base metal is internally oxidized in such a manner that the heating temperature in a reducing furnace is determined by a formula given by the partial pressure of steam and the dew-point temperature is increased. However, since an area for controlling the dew-point temperature is intended for the whole furnace, the control of the dew-point temperature and stable operation are difficult. The manufacture of a galvannealed steel sheet under the unstable control of the dew-point temperature causes the uneven distribution of internal oxides formed in a base steel sheet and may possibly cause failure including uneven plating wettability and uneven alloying.
PTL 3 discloses a technique in which coating appearance is improved in such a manner that a surface layer of a base metal is internally oxidized just before plating and is inhibited from being externally oxidized by regulating not only the concentrations of H2O and O2, which act as oxidizing gases, but also the concentration of CO2. In the case where a large amount of Si is contained as disclosed in PTL 3, the presence of internal oxides is likely to cause cracking during machining, leading to a reduction in exfoliation resistance. A reduction in corrosion resistance is also caused. Furthermore, there is a concern that CO2 causes problems such as furnace contamination and changes in mechanical properties due to the carburization of steel sheets.
Recently, high-strength galvanized steel sheets and high-strength galvannealed steel sheets have been increasingly used for parts difficult to machine and therefore exfoliation resistance during heavy machining has become important. In particular, in the case of bending a plated steel sheet to more than 90 degrees such that the plated steel sheet forms an acute angle or in the case of machining the plated steel sheet by impact, the exfoliation of a machined portion needs to be suppressed.
In order to satisfy such a property, it is necessary to achieve a desired steel microstructure by adding a large amount of Si to steel and it is also necessary to highly control the microstructure and texture of a surface layer of a base metal lying directly under a plating layer which may crack during heavy machining. However, such control is difficult for conventional techniques; hence, a galvanized steel sheet with excellent exfoliation resistance during heavy machining has not been capable of being manufactured from a Si-containing high-strength steel sheet in a CGL equipped with an annealing furnace that is an all-radiant tube-type furnace.